Aeroplanes operate in very low temperature environments, mainly due to the altitude which they fly but also due to low temperatures on the ground in colder climates. Especially, extreme conditions may occur at high altitude in Polar Regions. As the fuel temperature decreases, the viscosity increases, and may reach a critical viscosity where the fuel becomes difficult to atomise. This may cause the fuel to be difficult or even impossible to ignite. Moreover, the fuel in the fuel tanks may during flying be cooled down to a temperature at which the viscosity of the fuel exceeds a pumpability limit, thus making it impossible to deploy the fuel to the engines.
Thus, it is common practice to select the aviation fuel by one or more standardized criteria set by specification and standardization authorities for aviation fuels. The criterion is that the viscosity of the aviation fuel must be below a predetermined level when measured at a temperature of e.g. minus 20 degrees Celsius.
It has surprisingly been found that drying the inner surfaces with compressed air so as to remove any solvent used to clean the viscometer between tests, is not sufficient to obtain a valid measurement. The reason is that although the solvent has been removed, air contained inside the capillary assembly during cooling thereof, causes dew to be formed on the inner surfaces of the capillary assembly as the moisture in the air reaches its dew point and condenses on the inner surface of the capillary tube. Moreover, the dew may form ice crystals on the inner surface of the capillary assembly when the temperature is lowered to a temperature below zero degrees Celsius. The formed ice makes the measurements with the capillary assembly unrealisable.
Furthermore, it has surprisingly been found that using a test for filterability is not suitable for determining quality of e.g. an airplane fuel, as a sample with high cloud and pour points may block a filter ahead of a more viscous sample which contains little or no wax and consequently does not block the filter.
Accordingly, it is an object of embodiments of the present invention to provide an improved method for determining whether the aviation fuel is suitable for use e.g. in extreme conditions.
Moreover, it is an object of embodiments of the present invention to prevent formation of dew and ice crystals on the inner surface of the capillary assembly.